Secret, composed of a folding framework of four steel arched bands, pivoted at the junction in the centre, and connected by two sets of smaller cantilever ribs. When expanded it was worn inside the crown of a felt hat. It folds up into a compact shape that can be placed in the pocket.

With the growing disuse of armour many cavalrymen preferred to wear a felt hat, and these were often reinforced by steel caps or 'secrets' inside them. It is recorded that King Charles I always wore a felt hat in battle. Similar skeleton-caps are in the Musée de l'Armée (H 291-3), and one is in the Military History Museum, Brussels (II, No. 194, Fig. 31). Other examples are in the Musée de l' Armée, Paris (No. H.Po.1386 - previously in the Pauilhac collection) and one in the Royal Armouries (IV.582 - previously in the collection of Dr. Richard Williams). Further examples were in the Uboldo (1869) and Morosini Collections (sold New York, 1932, lot 437). One belonging to Mr. Bernhard Smith is illustrated in Helmets and Mail, 1880, p. 541, No. 118. Another, retaining what is apparently its original blueing, is in the Hermitage Museum, Leningrad (Z.O. No. 3314).